DD211010A1 - FIELD LIMITING LAYER FOR PREFERRED POLYETHYLENE MEDIUM VOLTAGE CABLE - Google Patents

Abstract

Field boundary layer f. preferably polyethylene-medium voltage cables, Leiterglaettung, insulation and leitfaehige field limitation of the cable are applied in the same operation. The aim of the invention is that the outer conductive layer of the core wrapping can easily be peeled off both in the uncrosslinked and in the crosslinked state. Taskgemaess is d. By addition of a polar thermoplastic d. Adhesive strength of the field limiting layer controlled so that it increases only slightly in the uncrosslinked and crosslinked state. According to the invention, the conductive compound consists of 100 parts of ethylene vinyl acetate copolymer (EVA) with a vinyl acetate (VA) content between 5% and 30%, preferably 10 to 12% 10-300 parts of an EVA grafted with vinyl chloride (VC) with a VA Proportion between 10% and 25%, preferably 10 to 12%, and a VC content between 10% and 60%, preferably 10 to 20% 60-250 parts Leitruss.

Description

Field boundary layer for preferably polyethylene medium voltage cable

Field of application of the invention

The invention relates to a field boundary layer for preferably medium voltage polyethylene cable, wherein ladder smoothing, insulation and conductive field limitation of the cable in the gleiohen operation and usually the ladder smoothing and isolation in tandem and the outer field boundary by co-extrusion by means of a Doppelspritzkopfes are applied.

Characteristic of the known technical solutions

For the production of terminations, branch and coupling sleeves, it is necessary that the outer field boundary layer can be removed without great effort without residue from the insulation. On the other hand, it is also a condition that this layer "electrically" so firmly adheres to the insulation that they can fully fulfill their function as a field boundary layer. These inherently contradictory requirements of "electrical welding" of outer conductive layer and insulation on the one hand and easy peelability on the other hand lead to a significant confinement of the materials in question · In order to enable the peeling of the conductive Aderschirmschicht of the insulation are Several methods have already been tried with varying degrees of success, such as, for example, coating the insulation or coating the insulation with release agents before applying the conductive shielding layer.

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Other methods are based on the use of release agents, which are incorporated directly into the conductive polymer (DB-OS 27 23 488). It is also known the influence of Vinylätätanteils of A'thylenvinylacetat (BVA) on the adhesion to polyethylene and it is given as the cheapest base resin an EVA with a vinyl acetate content of 27-45 wt .- #, with increasing vinyl acetate content, the adhesive strength the PB is reduced (DB-OS 26 20 105).

Furthermore, the use of conductive intermediate layers is described, the mechanical strength of which is smaller than that of the insulation and smaller than that of the outer conductive layer (DE-OS 24 30 792).

In any case, the conductivity of the Misohung is achieved in a known manner by mixing Bess. All the methods described hold the power that the proposed materials do not meet the specific requirements of radiation crosslinking in many respects.

Although low-VA-type EVA types are already good at low-dose radiation, they have the disadvantage of too high adhesion to PE and are therefore no longer peelable. Higher-grade EVA types also crosslink well, but tend to lift off during irradiation Aufsohäumung the conductive layer by liberated Radiolysegas. Known in this context is a conductive compound of Äthylvinylacetat, thermoplastic polyurethane and Leitruß, the desired adhesion is duroh addition of the polar thermoplastic polyurethane controlled (DD-PS 155 663). Although this compound is suitable for the radiation crosslinking of cables, but a disadvantage is the fact that after radiation crosslinking, the adhesive strength of the field boundary layers increases sharply, so that, in order not to exceed certain V.'erte for the adhesion of field boundary materials are used which has a very low adhesive strength in the uncrosslinked state

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Due to this low adhesive strength, these materials are not suitable as a field-limiting layer for uncrosslinked medium voltage cables, since over-peeling can lead to defects.

Moreover, all field boundary layers can be used either only for uncrosslinked or only for radiation crosslinked cables.

Object of the invention

The solution according to the invention should make it possible, especially for the voltage range from 6 kV to 30 kV, to easily peel off the outer conductive layer of the core coating both in the uncrosslinked and in the crosslinked state for assembly purposes and to adhere to the insulation without cavities.

Explanation of the essence of the invention

The invention has for its object to provide a field boundary layer whose adhesive strength is controlled on the insulation by the addition of a polar thermoplastic so that it is kept in both the uncrosslinked and crosslinked state in a tolerable tolerance range. The features of the invention are that the conductive compound

100 parts of ethylene vinyl acetate copolymer (EVA) with a vinyl acetate (VA) content between $ 5 and $ 30, preferably $ 10 to 12 #

10-300 parts of vinyl chloride (VC) grafted EVA having a VA content of between 10 # and 25 #, preferably $ 10 to $ 12, and a VC content of between 10 ^ 5 and 60 $, preferably 10 # to 20 # 60 - 250 parts carbon black

and other additives such as lubricants, stabilizers and crosslinking activators up to 10% by weight. Appropriately, it is assumed that a low to medium percentage A'thylenvinylacetat copolymer. By adding suitable strongly polar graft polymers of vinyl chloride on A'thylenvinylac et at copolymer to the base polymer, the adhesive strength can be controlled according to requirements.

Surprisingly, it is achieved by suitable formulation that the adhesion strength increases only slightly by the radiation crosslinking, whereby the use of the material according to the invention for uncrosslinked as well as crosslinked Feldbegrenzungssohichten is possible. The required degree of crosslinking is achieved owing to the easy crosslinkability of the ethylenevinylacetate. The grafted vinyl chloride content and the use of a BVA with the lowest possible VA content as the base polymer achieve high heat resistance, which counteracts foaming or blistering as a result of radiolysis gases released by the irradiation , The invention will be described in more detail using an exemplary embodiment.

embodiment

A conductive shielding from a mixture of

100 parts EVA (VA content: I, 100 parts EVA (VA content: 10 #), which with

$ 15 VC was grafted 95 parts carbon black

is co-extruded onto polyethylene insulation. The adhesive strength of the field confining layer is 8 H / cm. It can be removed without residue from the insulation.

After subsequent crosslinking with a radiation dose of 20 μm rad, an adhesion strength of the light-limiting layer of 30 M / cm is measured on the same product. When removing the conductive layer, no residues remain on the insulation. The degree of networking (hot-set test) according to IEC 20 A (Central Office) 30 is $ 95.

Claims (2)

    · - ~ 5 -. ' , Invention claim: 1 field limiting layer for preferably polyethylene medium voltage cable, characterized in that the conductive compound 100 parts of ethylene vinylaoetate copolymer (BVA) with a vinyl acetate (VA) content between 5 $ and 30 $, preferably 10 to 12 $ 10 - 300 parts of a vinyl chloride (VC) grafted BVA with a VA share of between $ 10 'and $ 25, preferably $ 10 to $ 12, and a VC share of between $ 10 and $ 60, preferably $ 10 to $ 20 60 - 250 parts of conductive black consists. 2. Peldbegrenzungsschicht according to item 1, characterized in that the compound contains stabilizers, lubricants and crosslinking activators up to 10 Gewe- $. ι>